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(1) Field of the Invention
This invention relates to a dual-beam materials-processing system for applying two collinear and coincident controlled pulses of radiation to materials for making physical changes; and more particularly, relates to a means for obtaining high-fluence patterned irradiation that is highly uniform, and highly controllable in shape, intensity, and pulse duration to achieve high resolution imaging for materials processingxe2x80x94without feeding a high fluence through the imaging system.
(2) Description of Related Art
Several techniques have been developed for converting thin amorphous silicon films into polycrystalline films. See Im and Sposili, Materials Research Society Bulletin, 21, 39 (1996). This is desirable because better-performing thin-film transistor (TFT) devices can be fabricated from crystalline silicon than from amorphous silicon, and it is not possible to deposit silicon films directly in a high-quality crystalline state. Among the various crystallization techniques, those based on excimer laser irradiation (or pulsed-laser irradiation more generally) are prevalent because they are compatible with substrates that cannot withstand high-temperature processing; the techniques can be applied when the silicon film to be crystallized is deposited on such a substrate. Glass and other substrates that cannot withstand high-temperature processing (e.g., plastics) are required for use in many TFT applications, for example, in displays.
Sequential lateral solidification (SLS) is a particular type of excimer laser crystallization process that can produce previously unavailable large-grained and grain-boundary-location-controlled microstructures in thin silicon films. The technique is well documented; see Sposili and Im, Applied Physics Letters, 69, 2864 (1996); Sposili and Im, Applied Physics A, A67, 273 (1998); Im, Sposili et al., Applied Physics Letters, 70, 3434 (1997); Sposili, Crowder et al., Mater. Res. Soc. Symp. Proc., 452, 953 (1997); and Im, Crowder et al., Physica Status Solidi A, 166, 603 (1998). Films having these microstructures are superior to the types of random-microstructure polycrystalline silicon produced by other excimer laser crystallization processes (and by non-excimer-laser-based crystallization processes as well) in that the TFTs exhibit a combination of superior electronic performance (e.g., higher carrier mobility) and a high level of device-to-device uniformity. The availability of high-performance TFT devices enables numerous applications, such as integrated active-matrix liquid-crystal displays (IAMLCDs), where the driver circuits and other electronics are integrated directly onto the substrate along with the pixel-controlling transistors, and active-matrix organic light-emitting displays (AMOLEDs), among others.
SLS requires that the silicon film be completely melted in and only in a micron-sized spatially controlled region or regions with each irradiation, and that the film be translated with sub-micron precision in between irradiations such that the lateral crystallization induced by each irradiation overlaps with that produced previously. Various schemes have been proposed for conducting the SLS process, and different approaches exist for effecting the requisite spatial tailoring of the beam. Projection irradiation is a very flexible method, wherein a patterned mask is imaged onto the film in order to define the location and extent of the molten zones. Generally, any projection SLS system would contain the following elements: a pulsed-laser source, typically an excimer laser; an illumination system, including a homogenizer, to provide uniform illumination of the mask; an imaging system to image the mask pattern onto the film; and a high-precision sub-micron sample-translation system.
Such SLS projection systems are similar in many respects to photolithography and ablation systems based on excimer lasers; the basic components listed above are common to all. While the various components and subsystems have the same general purpose for each type of projection system, the requirements of the processes differ and therefore so does the configuration of the subsystems.
An excimer laser projection system designed for SLS requires a high fluencexe2x80x94sufficient to completely melt a silicon film in the exposed region or regions. Unlike in lithography and ablation, in which the desired material changes can be effected equally well whether the required dose is delivered in a single irradiation or distributed over several irradiations, the physics of melting and solidification involved in the SLS process requires that the dose be delivered in a single irradiation. As a consequence of this fact, combined with the high energy density needed to heat silicon above its melting point of approximately 1,400xc2x0 C., successful execution of the SLS process requires that the films be irradiated at very high fluence.
The dual requirements of high fluence and projection imaging with micron-scale patterning present an engineering challenge because the mask materials commonly used, such as chrome-on-quartz, cannot withstand the level of fluence required to melt the silicon films. Conventional SLS system designs use demagnification to address this difficulty; the projection lens is designed to demagnify the image of the mask features. In this way, the fluence at the mask plane can be kept below the mask-damage threshold while achieving high fluence at the silicon film. Neglecting optical losses in the imaging system, the fluence at the silicon film (image plane) will be greater than the fluence at the mask plane (object plane) by a factor of the demagnification squared.
In a previously submitted patent, we described a number of innovations for SLS equipment including a novel illumination subsystem, which produces a self-luminous beam of selected cross-section, spatially uniform intensity, and selected numerical aperture; and a high-efficiency energy-recycling exposure system, which extends the duration of radiation pulses and conserves energy. The importance of these innovations can be understood in connection with the high-fluence requirements for SLS, and additionally in connection with the requirement that such high-fluence irradiation be spatially homogenous within the irradiated regions. If the intensity is too low in particular regions, the film will not melt completely, leading to failure of the process; if the intensity is too high, film damage can occur. In the previous patent, these inventions were described in the context of a conventional SLS system, of the type that utilizes demagnification imaging to keep the fluence level at the mask plane below the damage threshold of the mask materials. However, the utility of the inventions described in the previous patent is not limited specifically to the conventional SLS system design; rather the benefits of the previous invention are equally applicable to the type of system described herein.
Although demagnification is commonly used to address the fluence-limitation issues of SLS equipment, there are advantages to conducting the SLS process using equipment modeled after Anvik""s large-area, 1:1-magnification patterning systems rather than using the conventional reduction design.
Anvik""s large-area patterning technology is based on the Jain design [1-4] and is schematically illustrated in FIG. 1. The substrate 11 and the mask 12 are rigidly mounted on a single-planar scanning stage 13 that is capable of moving them in synchronism in both x- and y-directions. The illumination system 14 comprises an excimer laser light source 15 and a beam-processing optical system 16. The excimer laser 15 emits several tens of watts of UV radiation at 351 nm, a wavelength well-suited for imaging conventional photoresists designed for mercury arc lamp exposure. The systems can also be configured to use an excimer laser operating at 308 nm or 248 nm, depending on the application. The laser emits no infrared or other unwanted radiation. The beam-processing system illuminates the mask 12 from below (through a cutout in the stage) in a uniform, hexagon-shaped illumination region 17 that is typically 50 mm in size (vertex-to-vertex).
The mask pattern within the illuminated hexagonal region is imaged onto the substrate 11 by a unit-magnification projection lens 18 through a folded image path 19. The projection lens 18, the illumination system 14, and all other optical components are stationary, as are all the light rays. The sole moving component is the single-planar stage 13, which is scanned in a serpentine fashion in the x-y plane so that the following sequence of events happens: the mask 12 and substrate 11 scan in unison along the x-axis across their respective hexagonal illumination regions; the stage 13 moves them along y by an amount equal to the effective scan width w (typically 35-40 mm); the mask 12 and substrate 11 scan again along x (but in the opposite direction); they again move laterally by w along y, and the process is repeated until the entire substrate 11 is imaged. The hexagonal illumination configuration ensures that the whole exposure is completely seamless and uniform.
In order to understand the reason for the advantage of SLS systems based on the Anvik technology over conventional systems, it is necessary to consider that SLS is an intrinsically flexible crystallization method that can be implemented using many different schemes and configurationsxe2x80x94i.e., different beam patterns and irradiation/translation sequences. Depending on the details of the SLS scheme, the SLS process can produce a wide variety of low-defect-density crystalline microstructures, ranging from large-grained polycrystalline to single-crystal islands. As is well known, the performance of electronic devices depends very strongly on the microstructure of the semiconductor on which they are fabricated. Consequently, different SLS schemes will provide varying levels of device performance depending on the microstructural result.
In addition to determining the microstructure, the choice of the SLS scheme will also affect important processing characteristics, such as the throughput. In general, the selection of the SLS scheme for a particular application will be based primarily on the microstructural requirementsxe2x80x94as determined by the requisite level of device performancexe2x80x94and secondarily on the optimization of the process, particularly the throughput.
It is important to note also that the choice of SLS scheme is closely tied to the configuration of the equipment: Certain equipment designs are better suited for implementation of particular schemes. The conventional (demagifying) SLS system, in conjunction with certain SLS schemes, is well-suited for producing a particular large-grained, directionally solidified polycryststalline microstructure known as xe2x80x9cSLS-poly,xe2x80x9d and for doing so at high throughput. This microstructure provides a moderate level of device performance that is expected to be adequate for current TFT applications.
However, it is anticipated that future applications will require a higher level of performance than the SLS-poly microstructure can provide, and no scheme has been conceived for producing microstructures associated with better device performance at high throughput using the conventional SLS equipment. In contrast, the Anvik-style system, used in conjunction with SLS processing schemes that are not compatible with conventional equipment, and which will be described in a later section, can provide superior material at high throughput.
Unlike the conventional design, this technology requires that the imaging be done at 1:1 (i.e., no demagnification). However, the high fluence at the mask plane poses a serious problem for the implementation of 1:1 imaging for SLS. The high fluence required to melt the silicon films will damage commonly available mask materials; even the most resistant mask materials currently available are not expected to survive a large number of irradiations at such high fluence.
Accordingly, there exists a need in the field for a 1:1 projection imaging system that is compatible with Anvik""s large-area patterning technology and that can deliver high fluences at the image plane (e.g., as required for SLS) at the requisite level of imaging resolution without requiring that the mask plane be subjected to similarly high fluences.
The invention is a system for materials processing through projection irradiation using a pulsed laser sourcexe2x80x94particularly for conducting the sequential lateral solidification (SLS) process on substrates, including large-area substrates, containing a thin surface layer of material to be crystallized, typically but not limited to amorphous silicon (a-Si). The system is capable of generating a high-resolution image at high fluence without subjecting the imaging system itself to high fluence. It does so by generating two beams, one of which is passed through an imaging system while the other is introduced as uniform flood irradation over the imaging field. The superposition of the two beams produces an imaged beam of high fluence. The material, commonly referred to as the substrate, is made available at a work station for processing by a patterned laser beam. The system typically includes mechanisms for positioning and translating large-area substrates. The positioning generally includes a schedule of scanning and stepping that is coordinated with the irradiation in a manner consistent with the requirements of the particular process being conducted. During the scanning motion, the system at the same time provides controlled laser pulses, modifying the laser radiation in two separate illumination subsystems, and projecting patterned, modified laser radiation along one of the two optical paths to a carefully controlled area on the surface of the substrate.
The desire is to create desirable microstructures in the film, such as large-grained, grain-boundary-location-controlled polycrystals and large single-crystal islands at selected locations on the substrate.
The output of a single laserxe2x80x94a train of pulses controlled in energy and repetition ratexe2x80x94is split in two by a variable beam-splitter (VBS), which allows for adjustment of the split of the beam energy between the two sub-beams. Alternately, two lasers are used to generate two separate beams of different wavelengths. Both of the two optical paths contain illumination systems comprising three modular units: a set of anamorphic beam-shaping optics, a homogenizer, and a set of condenser lenses. The illumination systems reshape and homogenize the beam, and provide uniform self-luminous illumination in the desired shapexe2x80x94typically a high-aspect-ratio rectangle. The homogenizer is a tubular structure with fully mirrored walls and a polygonal cross-section. The many mirror surfaces of the homogenizer provide multiple reflections; the caroming about of the beam within the homogenizer converts the incident beam into spatially homogenized, self-luminous illumination at the output aperture. The homogenizer may also incorporate Anvik""s energy-recycling technology, which recovers energy reflected off of the mask that otherwise would have been lost, and reintroduces it into the illumination. The energy recycler also extends the laser pulse duration by 15-30%; otherwise-wasted laser power is conserved, and there is an opportunity to control pulse duration quite effectively.
Along the imaging path, the uniform illumination is imaged onto a mask, which in turn is imaged by a projection lens onto the sample. In the non-imaging path, the uniform illumination is imaged directly onto the sample. Various schemes, such as having a physical or optical hole in the final folding mirror, or a dielectric coating that reflects one wavelength but transmits another, can be used to render the beams coincident as they impinge on the sample. The superposition of the two beams provides an image of enhanced fluencexe2x80x94equal to the arithmetic combination of the two beams. The system does so without subjecting the mask and imaging system to high fluence. Areas within the image field that do not contain any patterned features receive only the uniform illumination dose from the non-imaging branch. A significant portion of the high-fluence beam energy within the patterned areas is absorbed by the substrate and effects a physical change, usually through a thermal mechanism as a result of localized intense heating. However, other modes of interaction between the beam and the substrate are possible, depending on the specific substrate material and the wavelength of the laser radiation. The level of uniform illumination from the non-imaging branch will generally be adjusted so as to be below the threshold for the operative physical change, and may only serve to effect some amount of low-level heating of the film.
The localized intense heat is useful in many materials-processing operations, including those involving localized melting and resolidification, such as ELA in general and SLS in particular, and dopant activation, as well as those involving localized ablation and removal of material.
The object of the invention is to provide a high-fluence, shaped, spatially homogenized, duration-controlled, and carefully imaged radiation beam at the chosen area or areas of the substrate, which can be translated in an extremely precise and controllable manner in coordination with the irradiation, in order to accomplish the desired physical change for materials processingxe2x80x94particularly SLS.
Another object of the invention is to provide a high-fluence, high-resolution patterned radiation beam using 1:1 projection imaging (no demagnification) without exposing the mask and imaging system to high-fluence radiation.
Yet another object of the invention is to provide a practical system for conducting the SLS process using Anvik""s large-area 1:1 patterning technology.
A feature of the invention is the use of a novel optical system, which provides two beams from a single laser that irradiate the sample concurrently.
Another feature of the invention is the use of two lasers operating at different wavelengths. A low- to moderate-power laser feeds the imaging path of the system, and a high-power laser feeds the non-imaging path of the system.
Yet another feature of the invention is the confluence of two separate irradiation beams incident collinearly onto the sample.
An advantage of the invention is the ability to perform 1:1 image-based materials processing at very high fluence.
Another advantage of the invention is the prolonging of mask life as a result of the reduction in fluence through the imaging system.
Other objects, features and advantages of the invention will be apparent from the following written description, claims, abstract, and the annexed drawings.
References
1. K. Jain, xe2x80x9cScan and Repeat, High-Resolution Lithography System,xe2x80x9d U.S. Pat. No. 4,924,257, issued May 8, 1990.
2. K. Jain, xe2x80x9cA Novel High-Resolution, Large-Field, Scan-and-Repeat, Projection Lithography System,xe2x80x9d Proc. SPIE, Vol. 1463, p. 666, 1991.
3. K. Jain, xe2x80x9cLarge-Area, High-Throughput, High-Resolution, Projection Imaging System,xe2x80x9d U.S. Pat. No. 5,285,236, issued Feb. 8, 1994.
4. K. Jain, xe2x80x9cHigh-Throughput, High-Resolution, Projection Patterning System for Large, Flexible, Roll-Fed Electronic-module Substrates,xe2x80x9d U.S. Pat. No. 5,652,645, issued Jul. 29, 1997.